A modem (modulator-demodulator) is a data communications device which allows communication between digital computing machines over voice-grade telephone lines. Typically, in order to establish the communication, one modem, the originating modem, dials the telephone number of the location where another modem, the answering modem, is located. The answering modem detects the ringing signals on the telephone line and seizes the telephone line.
A first handshake procedure then follows. The answering modem places an answer tone on the telephone line or, if selected by the user, remains silent. The originating modem detects the answer tone, or silence, and places the originating data carrier on the telephone line. The answering modem detects the originating data carrier, removes the answer tone and places the answering data carrier on the telephone line. At the end of this first handshake procedure the two digital computing machines may begin exchanging data (log-on, procedures, identification codes, document transfer, etc.). This, of course, assumes that the two modems were programmed for the same speed (baud rate), mode (synchronous or asynchronous), and operating format (number of data bits, presence and nature of parity bits, type of synchronous format, etc.). If the speed, mode, and format do not match, then the data cannot be exchanged.
Some, more advanced, modems perform a second handshake procedure to select the proper speed. Typically, the originating modem starts at the highest speed for which it is capable. The answering modem determines this speed and, if possible, starts at this speed. If the answering modem cannot attain this speed, the answering modem starts at the highest speed for which it is capable. The originating modem determines the speed of the answering modem and, if different from its own speed, adjusts its speed downward to match that of the answering modem.
Also, many systems, in order to prevent unauthorized access, have a third handshake procedure, known as a logon procedure. A typical logon procedure requires the transfer of one or more identification numbers, access codes, etc. Some of the advanced modems can be programmed to perform this third handshaking procedure, which is a great convenience to the user.
While the above handshake procedures work well, they are limited in function and do not allow the modems to exchange other data which is useful, such as the type of device, modes supported, formats supported, signal quality, etc. It is, of course, within the state of the art to design a modem which can provide and act on this information. However, two problems arise.
First, the information must be transferred in a manner which does not cause this information to appear at the digital computing machines or interfere with the above handshake procedures. It is, of course, a simple matter to design a modem which initially blocks data transfer to and from the digital computing machine so that this information is not provided to the digital computing machine, and which transfers this information at a time which will not interfere with the above handshake procedures.
However, when a first modem which can perform such information transfer is connected to a second modem, the first modem must determine whether or not the second modem can perform such information transfer. Therefore, a fourth handshake procedure is required. However, this fourth handshake procedure will be passed on to the digital computing machine if the second modem is a standard modem which cannot perform such information transfer. Therefore, the second problem is providing a fourth handshake procedure which, when passed to the digital computing machine, does not affect the digital computing machine, or cause this fourth handshake procedure to be displayed.
There is therefore a need for a handshaking procedure which, when provided to a modem which is not equipped for said handshake procedure, does not adversely affect the modem, the digital equipment connected to the modem, or the standard handshake procedures.
There is also a need for a modem, with an improved handshake capability, which can communicate with modems which do not have said improved handshake capability.
There is also a need for a modem, with an improved handshake capability, which can exchange data with another modem with said improved handshake capability while preventing said data from being provided to digital computing machines connected to said modems.